Current control circuit



March 15, 1949.

w. B. MARTIN ET AL CURRENT CONTROL CIRCUIT Filed July 12', 1947 In 24 M/9 IIIIIIIIIIIIII! FICLZ INVENTORS Patented Mar. 15, 1949 CURRENTCONTROL CIRCUIT Wade B Martin, Ramsey, N. J and Harry Talberth,Brooklyn, N. Y.

Application July 12, 1947, Serial No. 760,630

.6 Claims.

This invention relates to a current control circuit and, moreparticularly, to an improved current control starting circuit for firinga fluorescent lamp.

A fluorescent lamp in general comprises a suitable mixture of inertgases contained within an elongated tubular envelope having its interiorsurface coated with a fluorescent substance. Widely spaced thermionicelectrodes of filamentary form are mounted within the tubular envelopenear opposite ends thereof. In lighting the lamp, it is first necessaryto ionize the gaseous atmosphere within the envelope by heating thelfilamentary electrodes to an electron emitting temperature because atroom temperatures the gas will not pass sufficient current for ordinarylighting purposes. This is accomplished by employing a current controlstarting circuit which permits the flow of a relatively high currentthrough the filamentary electrodes in series to preheat them to thepoint of electron emission.

When the temperature of the electrodes reaches this point, the currentcontrol circuit abruptly interrupts the flow of current thereby causinga relatively high firing potential to be momentarily applied to theelectrodes. This ionizes the gaseous atmosphere within the envelopewhich then serves as a conductive medium for sustaining a current flowbetween the electrodes at line potential. This line current flow throughthe gaseous atmosphere between the electrodes generates high intensityradiations within the invisible portion of the spectrum which, as isWell known in the art, excite the fluorescent coating on the interior ofthe lamp and cause it to fluoresce thereby producing visible light.

In designing a current control circuit for controlling the applicationof the preheating current and the firing potential, it is necessary thatthe circuit be designed to allow ample time for preheating theelectrodes before causing the firing potential to be applied asotherwise the electrodes would be damaged, the ends of the lamp would bedarkened, and considerable flickering and sputtering of the lamp wouldoccur. On the other hand, it is also desirable, for obvious reasons,that a starting circuit should be designed to consume no more time inthe preheating period than is necessary to raise the temperature of theelectrodes to an adequate emissive temperature. In attempting to strikea balance between insufficient preheating and unduly long preheating, itsometimes happens that a starting circuit will fail to fire the lamp.

Accordingly, it is an object of this invention to provide an improvedcurrent control circuit.

It is also an object of this invention to providean improved currentcontrol circuit for controlling the application of a firing potential toafluorescent lamp.

Another object of the invention is to provide an improved currentcontrol starting circuit for firing a fluorescent lamp smoothly andwithout flickering or sputtering.

Still another object is to provide an improved current control circuitfor effecting the preheating of the filamentary electrodes of afluorescent lamp the instant that the line voltage is applied to thelighting circuit of the lamp.

A further object is to provide an improved current control startingcircuit for eflecting adequate preheating of the filamentary electrodesof a fluorescent lamp and for causing a firing potential to be appliedto them.

An additional object is to provide an improved current control startingcircuit for causing a firing potential to be automatically andrepeatedly applied to the filamentary electrodes of a fluorescent lampand for automatically deactivating itself in the event the lamp fails tofire after a preassigned interval of time.

These and other objects of the invention are attained by means of animproved current con trol circuit having a heating element constitutedby a high resistance which is connected in series with an inductiveballast and the lamp electrodes. This high resistance is normallyvirtually shortcircuited by a shunt path which includes a resistancewire heating element. The resistance wire heating element is helicallywound around the high resistance and both of them are disposed inheating relationship to a normally closed bimetallic switch. As soon asthe line current is applied to the lighting circuit of the lamp, thecurrent will immediately flow through the shunt path in the startingcircuit and will thereby immediately start to preheat the filamentaryelectrodes.

During this time, the resistance wire heating element becomes hot andradiates heat to both the high resistance and the bimetallic switch. Thebimetallic switch and the resistance wire heating element are soproportioned that the switch will open at about the same time that theelectrodes become sufficiently preheated for a discharge to be initiatedacross the lamp. When the shunt path across the preheated highresistance is thus opened, a high voltage surge is produced across theelectrodes by the inductive bal last and the lamp should fire.

If, for various reasons, the lamp fails to fire, the line current nowapplied to the preheated high resistance will cause it to generate moreheat which is radiated to the bimetallic switch causing it to open stillfurther until it engages a second bimetallic switch which also has aresistance wire heating element connected to it in heating relationship.This establishes another virtual short-circuit across the highresistance. vCurrent will now flow through this second shunt 3 path tocontinue the preheating of the electrodes. During this time, the secondresistance wire heating element will become hot and will radiate heat tothe second bimetallic switch. Also during this time, the high resistancebecomes cool.

Thus, the heating of the second resistance Wire heating element tends todisengage the second bimetallic switch from the first bimetallic switchby moving the former in one direction while, at the same time, thecooling of the high resistance is conducive to disengagement of the twoswitches by permitting the first bimetallic switch to move in theopposite direction. The combination of these two opposite actions causesthe two bimetallic switches to separate quickly opening the second shuntpath across the high resistance. This "causes the inductive ballast toproduce a second high voltage surge across the electrodes which shouldfire the lamp.

If the lamp'stillfails to fire, the two bimetallic switches will againmove into "engagement with each other "and the sequence of operationsdescribed in the preceding paragraph will be repeated until the ambienttemperature within the housing structure of the starting circuit risesto a sufiiciently high degree to hold the bimetallic switches in theirextreme positions, as is described in "more detail hereinafter, thereby,preventing further recycling operations.

Tnese'and other features of the invention'are 7 "mo'ie'fully dicussed'in connection with the fol-- lowing detailed description of thedrawing in which:

Fig. lis'a schematic circuit diagram of the improved current controlcircuit, and

Fig. 2 is a front perspective view with the housing structure brokenaway to 'show'the compact manner in'which the components of the improvedcurrent 'c'o'ntrol'starting circuit are assembled.

, In Fig.'1;a fluorescent lamp l containing a suitable mixture of inertgases is shown to be pro- "vided with oppositely disposed filamentaryelectrod'es2'and 3 near each end thereof. A supply source of thecustomary domestic potential, such I "as apower line supplying'115 volt60 cycle current, isjindicated by the reference numeral 4 foroperatin'g'the lamp I. Qne side of the power supply source '4 isconnected through a suitable ballast impedance, such as a choke coil 5,'tothe electrode 3. The other side of the power source 4 is "connectablethrough a manually operable control switch 6'to the electrode 2. Theelectrodes 2 and 3 are connected in series through a suitable high"resistance 'l'which also functions as a heating element in the currentcontrol circuit as is described hereinafter. A current control startingcircuit 8 'for'con'trolling the application of both the preheating cur-*remram the firing potential to the electrodes 2 "and 3 is connectedacross the high resistance I in "shunt relation thereto. Thiscurrentcontrol cir- "cuit' 8 includes a resistance wire heating element'9 which is helically 'wou'ndaround' the high resistance has is showninFig.'2,'forefficiently radiating heat thereto. Ascanfurthe'r be seenin Fig. 2, both the 'highreslstance l and theresiste'nce wire neatinelement 9 are disposed in heating relationship to a-'fiei iblebimetallic switch Hi composed of any two suitable metals havingdissimilar 'thermalexpansion 'coeiiicients. One end of the "bimetallicswitch I is fixedly f mounted at the pbir'it l I.

' Nearthe othen'end of the bimetallic "switch I0,"a metalliccontact rodM' isafiixedto one side thereof byany convenient means, such asq'o'ywelding. "Thei-endof the contactirod 14 is bent at an angle for movementagainst a stop provided by a mounting card indicated schematically at24.

Another flexible bimetallic switch 16 composed of any two suitablemetals having dissimilar thermal expansion coefficients has one endfixedly mounted at the point I! in electrically conductive engagementwith one end of the resistance wire heating element 9. A second metalliccontact rod I8 is aflixed, as by welding, to the bimetallic switch Hinear its other end for engagement with the first contact rod l4.

When the starting circuit 8 is in an unenergized condition, as is shownin Fig. 1, the contact rods I4 and I8 are in engagement with each other.

'Since the resistance wire heating element *9 "has a much smallerresistivity than that of the high thermal expansion'coeffici'ents hasone end'fixedly mounted at the point 20. Theother end-of the switch isis bent-upward at an angle-to form a contact lip 2| 'for engagement withthe contact rod Hi. Whenthe starting "circuit 8'is in anum energizedcondition, as is shown in Fig. '1, the contact lip 2| does not engagethe contact rod I4. A'second'r'esistance wire heating element-22 isdisposed in heating'relationship to the bimetallic switch l9 and has oneend connected-in electrically conductive engagement with the switch I iiat the point 20.

To operate this current control circuit, the manually operablecontrol'switch 6 isfirst closed to apply the line current from "thesource 4 to the inductive ballast 5 in series with the electrodes 2 and3 and the resistance wire heating element 9 in the shuntpath'acrossthehighresistance i. This flow of currentthrough the electrodes 2 and 3causes them to become warm thereby partially ionizing the gaseousatmosphere contained within the envelope of the lamp I. At the sametime, the fiow of current through the resistance wire heating element 9causes it to become hot.

The heat thus generated by'theresistance wire heating element 9 isradiated tothe bimetallic switch iii and also to the highiresistanc'e 1which both become warm dueit'otheir proximity which is more evident inFig.2. The'bimetallic switch Iii finally becomes so hot that'it bendsdownward thereby opening the circuitthrough the contact rods M and I8.As was stated above, the resistance wire heating element 9 'and thebimetallic switch in areso proportioned that the contact rods M and I8willseparate at about th'esarne time that the electrodes '2 and 3 becomesufiicie'ntly preheated for a discharge to be initiated across the lampl'.

The sudden iopening of the-circuit through the contact rods l4 and I8opens the shunt path across thehigh resistance '1 thereby, in *e'iiect,switching it in series with the inductive ballast Sand theelectrod'es 2"and 3. 'This resistance 7! has such a high resistivity that the suddensame effect as opening the series circuit"through the inductive ballast5 and the electrodes 2 and 3.

This interruption causes the inductive ballast 5 to produce a highvoltage surge or kick across the preheated electrodes 2 and 3 therebycom pleting the ionization of the gaseous atmosphere contained withinthe envelope of the lamp I and resulting in the striking of a lowvoltage are which, when once started, will be sustained by the linevoltage with the electrodes 2 and 3 being maintained at a dischargesustaining temperature by the discharge current through the lamp I. Asmall condenser 23 is connected across the high resistance I to minimizesparking or arcing when the contact rod I4 moves away from the contactrod I8 to open the shunt path across the high resistance I.

During the time that the lamp I remains in operation, a small amount ofcurrent will flow through the preheated high resistance 1 to maintain itwarm. The heat now generated by the high resistance I is radiated to thebimetallic switch I and keeps it sufficiently warm to maintain thecontact rod I4 in an open position approximately midway between thecontact rod I8 and the contact lip 2| but not suificiently downward toengage the mounting card 24. The above-mentioned shunt path in thestarting circuit 8 thus remains open as long as the lamp I is inoperation.

It should be noted that, because of its relatively large mass, the heatinertia of the high resistance I is great. However, this inertia isovercome during the initial attempt to fire the lamp I due to the highresistance I having been first preheated by heat radiated by theresistance wire heating element 9.

In the event that the lamp I should fail to fire, a larger amount ofcurrent will flow through the high resistance 1. This current will havea voltage about double that which it would have had if the lamp I hadfired. The resistance I will therefore become hotter than it would havebeen under the conditions described in the preceding paragraph and, dueto its proximity as shown in Fig. 2, will radiate a relatively highamount of heat to the bimetallic switch IO. lhe bimetallic switch I 0now becomes so hot that it bends further downward until its contact rodl4 engages the contact lip 2I to close an auxiliary shunt path acrossthe high resistance I.

Since the resistance wire heating element 22 in this shunt path has aconsiderably smaller resistivity than the high resistance I, most of theline current will now flow through this path instead of through the highresistance I which will now become cooler and will cease to radiate asmuch heat to the bimetallic switch In so that it also will becomecooler.

This flow of current through the resistance wire heating element 22causes it to produce heat which is radiated to the bimetallic switch I9due to their proximity as shown in Fig. 2. This heat tends to cause thebimetallic switch I9 to bend downward. During this time, the cooling ofthe high resistance I tends to permit the b metallic switch III to coolas was described in the preceding paragraph so that it be ns to bendupward. Due to these simultaneously opposite actions, this auxiliaryshunt path is quickly opened to again place the high resistance 1 inseries with the filaments 2 and 3. As was stated above. this in effectinterrupts the series circuit through the induct ve ba last and theelectrodes 2 and 3 due to the high value of resistance I. The highvoltage surge now pro- 6 duced by the ballast coil 5 due to thisinterruption is applied to the preheated electrodes 2 and 3 in anotherattempt to fire the lamp I.

This sequence of operations is quickly repeated several times ifnecessary. If the lamp I fails to fire during these operations, theambient temperature of the air inside the housing structure, whichcompletely encloses the starting circuit 8 as is shown in Fig. 2,finally rises to a point sufi'iciently high to cause all threebimetallic switches l6, Ill, and I9 to bend further downward. Thebimetallic switch I9 is free to bend far downward so that its contactlip 2i will be out of engagement with the contact rod I4. The downwardbend of the bimetallic switch I0 is arrested due to the bent end of itscontact rod I4 hitting against the stop constituted by the mounting card24. Similarly, the downward bend of the bimetallic switch I6 is arresteddue to its bent end striking against the stop constituted by themounting card 24. Since the distance between the mounting card 24 andthe tip of the bimetallic switch It is shorter than the distance betweenthe mounting card 24 and the tip of the contact rod I4, the contact rodl4 will now bend down further than the bimetallic switch I6. Thisprevents the contact rod I c from coming into engagement with thecontact rod M at this time.

Thus, both the main shunt path across the high resistance 1 and theauxiliary shunt path across the high resistance i will now be held opendue to the heat inside the housing structure holding the threebimetallic switches I6, l0, and I9 in the downward positions justdescribed. The starting circuit 8 will remain in this condition due tothe heat produced by the flow of line current through the highresistance I. With both the shunt paths being thus held open, no furtherattempt to fire the lamp I will be made until the control switch 6 hasbeen opened to discontinue the fiow of current through the highresistance I and until the ambient temperature within the housingstructure becomes sufficiently reduced to permit the bimetallic switchesI6, I0, and I9 to move up out of their extreme downward positions.

It is to be noted that the reason for the stops provided by the bentends of the contact rod I4 and the bimetallic switch I6 hitting againstthe mounting card 24 is not only to prevent the contact rod I 8 fromengaging the contact rod I4 at this time but also to prevent the contactrod I4 from being carried down far enough to engage the contact lip 2Iduring this period of high ambient temperature.

In Fig. 2, the current control starting circuit 8 of Fig. 1 is shown tobe mounted on a card 24 of insulat ng material and is also attached to abase 25 of insulating material. The base 25 together with a cylindricalcasing 26 of any suitable material constitute a housing structure whichcompletely encloses the start ng circuit 8. The base 2515 provided withterminals 2! and 28 for facilitating the electrical connection of thestarting circuit 8 to the lighting circuit of the lamp I of Fig- 1. Itcan be seen in Fig. 2 that the high resistance I is connected across theterminals 27 and 28 and that the condenser 23 is also connected acrossthe terminals 2! and 28 in shunt with the resistance I.

The b metallic switch I0 is shown to be bent in the form of a J and isattached to the mounting card 24 at the point I I by any suitable means,such as by a rivet. It is electrically connected to to efficientlyradiate heat thereto.

the terminal 28 by a conductor 29 which passes through. an eyelet 33),along the back of the mouhtirg card it, and then through another eyelet35. The free end of the bimetallic switch iii welded thereto one end ofa metallic contact rod M which has its other end bent at an angle forserwing as a stop arm for the purpose discussed above in the descriptionof Fig. l.

The contact rod M is normally in contact with another metallic contactrod l8 welded near one end of the bimetallic switch it. The bimetallicswitch it is also bent in the form of a J and is attached to themounting card M at the point I! in a manner similar to that of thebimetallic switch iii. The other end of the bimetallic switch iii isbent at an angle to serve as a stop arm for the reason described above.The fixed end of the bimetallic switch H has soldered thereto one end ofthe resistance wire heating element 9 wh ch is helically wound aroundthe Bakelite shell of the high resistance l in order The other end ofthe resistance Wire heating element 9 is soldered to the conductor 3!which is connected to the terminal 27.

It is to be noted that both the high resistance l and the resistancewire heating element 9 pass through the bend at the bottom oi thebimetallic switch ill in order that the heat which they generate may bemore eificiently radiated to the bimetallic switch Hi. It is especiallyto be noted that, due to the resistance wire heating element ll beingsuperimposed upon and wound around the high resistance 1, the heatgenerated by the resistance wire heating element El will be efficientlyradiated to the high resistance '5 for preheating it as was discussedabove in the description of Fig. l.

The bimetallic switch H; has one end bent in the form of a J which isafiixed to the mounting card 2& at the point 28 in a manner similar tothat of the bimetallic switch H3. The other end of the bimetallic switchit is bent upward to form a contact lip 2| for engagement with thecontact rod i i. If desired, instead of thus bending the end of thebimetallic switch 19, a short section of contact rod material may besoldered along the top edge of the bimetallic switch I9 for performingthe same function as the contact lip 2i. resistance wire heating element22 which has one end soldered to the lead-in conductor 3| and has itsother end soldered to the fixed end of the bimetallic switch l9.

In order that the heat generated by the resistance wire heating element22 will not be effectively radiated to the other bimetallic switches inand iii, the wire 22 is passed from the back of the card M through aneyelet 32, then along the front of the card 26, through another eyelet33, along the back of the card 24, and then through another eyelet it tothe fixed end of the bimetallic switch I9.

The eyelets 32, 33, and 3 4 also perform a spacing function by reason oftheir rims projecting slightly out from the mounting card 24 and therebyserving to space the wire 22 a short distance away from the mountingcard 24 so that it will not become burnt or charred when the wire 22becomes hot. In addition, these eyelets 32, 33, and 34 also function toabsorb some of the excess heat produced by the resistance wire heatingelement 22.

The J-bend at the fixed end of the bimetallic switch I6 is large enoughto provide 'sufilcient The bimtallic switch It is heated by the spacingbetween'the free end of the bimetallic switch it and the resistance wireheating element 22 so that it will not be efiectively heated thereby.this regard, it should be noted, as was stated above in the descriptionof Fig. 1, that the bimetallic switch It is heated only by the ambienttemperature within the housing structure constituted by the casing 26and the base 25.

It is to be noted that the bimetallic switches Hi, It, and I9 areself-compensating for normal changes in the ambient temperature withinthe housing structure or for changes in room temperature. This isbecause such temperature changes will efiect equal movement of the freeends of each of the bimetallic switches l0, l6, and I9 so that theirrelative positions with respect to each other will remain the same.

It is to be understood that the specific construction described aboveand shown in the drawing is for the purpose of explaining the nature ofthis invention and that various modifications may be made thereinwithout departing from the scope of the invention which is to be limitedonly by the claims appended hereto.

What is claimed is:

1. In combination, a source of electric current, a high resistance, anormally closed main shunt circuit connected across said highresistance, a normally open auxiliary shunt circuit connected acrosssaid high resistance, and means for applying current from said source tosaid high resistance and said main and auxiliary shunt circuits, saidmain shunt circuit having a normally closed first bimetallic switchconnected in series with a first resistance wire heating elementhelically wound around said high resistance for eficiently radiatingheat thereto and disposed in heating relationship to said firstbimetallic switch, and said auxiliary shunt circuit including a normallyopen second bimetallic switch and expediting means for expeditingmovement of said second bimetallic switch, said expediting meanscomprising a second resistance wire heating element connected in seriesin said auxiliary shunt circuit and disposed in heating relationship tosaid second bimetallic switch for radiating heat thereto.

2. In combination, a source of electric current, a fluorescent lamphaving filamentary electrodes, a normally open control switch, aninductive ballast, a high resistance having one end connected to one ofsaid electrodes and having another end connected to another of saidelectrodes, circuit means for connecting said control switch and saidinductive ballast and said high resist ance and said electrodes all inseries with said source of current, a normally closed main shunt circuitconnected across said high resistance, said main shunt circuit includingfirst and second contact means normally in engagement with each otherand connected in series with a first resistance wire heating elementadapted to generate and radiate heat upon the closure of said controlswitch, said second contact means including first bimetallic means, saidfirst resistance wire heating element being disposed in heatingrelationship to said first bimetallic means for radiating heat thereto,said first bimetallic means being adapted when hot to move said secondcontact means out of engagement with said first contact means foropening said normally closed main shunt circuit, said inductive ballastbeing adapted to produce a first high voltage surge between saidelectrodes upon the opening of said main shunt circuit across said highresistance, said high resistance being adapted to become hot and toradiate heat upon the opening of said main shunt circuit with saidcontrol switch closed, said high resistance being disposed in heatingrelationship to said first bimetallic means for radiating heat thereto,and a normally open auxiliary shunt circuit connected across said highresistance and including third contact means having second bimetallicmeans connected in series with a second resistance wire heating elementdisposed in heating relationship to said second bimetallic means forradiating heat thereto, said first bimetallic means being adapted whenheated by said high resistance to move said second contact means intoengagement with said third contact means for closing said auxiliaryshunt circuit across said high resistance, said second resistance wireheating element being adapted to become hot upon the closure of saidauxiliary shunt circuit for heating said second bimetallic means, saidfirst bimetallic means being adapted to move said second contact meansout of engagement with said third contact means after said auxiliaryshunt circuit is closed, said second bimetallic means being adapted whenhot to move said third contact means out of engagement with said secondcontact means for opening said auxiliary shunt circuit, and saidinductive ballast being adapted to produce a second high voltage surgebetween said electrodes upon the opening of said auxiliary shunt circuitacross said high resistance.

3. A combination in accordance with claim 9 and comprising in addition ahousing structure for completely enclosing said high resistance and bothsaid main and auxiliary shunt circuits, said first contact meansincluding third bimetallic means, and a stop member disposed within saidhousing structure, all of said three bimetallic means being adapted tomove their associated contact means toward said stop member upon anincrease in the ambient temperature within said housing structure, saidsecond contact means having a first stop instrumentality adapted to hitagainst said stop member for holding said second contact means out ofengagement with said third contact means, and said first contact meanshaving a second stop instrumentality adapted to hit against said stopmember for holding said first contact means out of engagement with saidsecond contact means whereby both said main and auxiliary shunt circuitsare held open.

4. A current control starting circuit for an electric gaseous dischargedevice having a plurality of filamentary electrodes, said startingcircuit comprising in combination a high resistance having one endconnected to one of said electrodes and having another end connected toanother of said electrodes, circuit means including a main shunt pathand an auxiliary shunt path for shunting said high resistance, switchingmeans having at least three switch arms for controlling said shuntpaths, the first and second of said switch arms being normally inelectrically conductive engagement for closing the main shunt path, thethird of said switch arms being normally out of electrically conductiveengagement with the other two switch arms for opening the auxiliaryshunt path, means for causing said high resistance to become hot and toradiate heat to the second switch arm, said second switch arm, beingadapted when hot to move into electrically conductive engagement withthe third switch arm for closing the auxiliary shunt circuit, said highresistance being adapted to cool and to cease radiating heat to thesecond switch arm when said auxiliary shunt circuit is closed, saidsecond switch arm being adapted to move away from the third switch armfor opening the auxiliary shunt circuit when heat ceases to be radiatedto it by the high resistance, and accelerating means for acceleratingthe opening of the auxiliary shunt circuit after it has been closed,said accelerating means comprising a resistance wire heating elementconnected in series with the third switch arm and disposed in heatingrelationship thereto and adapted to become hot when said auxiliary shuntcircuit is closed, said third switch arm being adapted to move away fromthe second switch arm for opening the auxiliary shunt circuit when heatis radiated to it by said resistance wire heating element.

5. In a current control starting circuit for firing an electric gaseousdischarge device having a plurality of filamentary electrodes with meansfor producing a first high voltage surge between said electrodes forfiring said device, circuit means for quickly producing a second highvoltage surge between said electrodes in the event said device fails tofire in response to the first high voltage surge, said circuit meansincluding two switch arms of bimetallic material, the first of saidswitch arms being adapted when hot to move into electrically conductiveengagement with the second of said switch arms and when cool to move outof engagement therewith, the second of said switch arms being adaptedwhen hot to move out of electrically conductive engagement with saidfirst switch arm, and heat radiating means adapted to radiate heat tosaid second switch arm only after said first switch arm has been heatedand is beginning to cool.

6. A current control starting circuit for an electric gaseous dischargedevice having a plurality of filamentary electrodes, said startingcircuit comprising in combination a high resistance having its first endconnected to a first one of said electrodes and having its second endconnected to a second one of said electrodes, 2. first bimetallic switcharm, a first resistance wire heating element having one end connected tosaid first switch arm and having its other end connected between thesecond end of the high resistance and said second electrode, a secondbimetallic switch arm, a second resistance wire heating element havingone end connected to said second switch arm and having its other endconnected between the second end of the high resistance and said secondelectrode, a third bimetallic switch arm having a fixed end and a freeend adapted to move into electrically conductive engagement with saidfirst and second switch arms alternatively, and circuit means forconnecting the fixed end of said third switch arm between the first endof the high resistance and said first electrode.

WADE B MARTIN. HARRY TALBERTH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,356,813 Betz Aug. 29, 19442,426,229 McCarthy Aug. 26, 1947

